How this calculation works
The Paint Calculator determines the exact paint volume (gallons and quarts) required to paint your walls and ceiling, factoring in wall perimeter, ceiling height, and door/window cutouts.
Mathematical formula and logic
Wall Area = 2 × (Length + Width) × Height - (Doors × 21 + Windows × 15). Gallons Needed = ceil((Wall Area × Coats) / 350 sq ft per gallon).
Worked example
A 14 × 12 ft room with 8 ft ceilings has 416 sq ft of gross wall area. Subtracting 2 doors (42 sq ft) and 2 windows (30 sq ft) leaves 344 sq ft net wall area. Applying 2 coats (688 sq ft total) requires 2 Gallons of paint.
Calculation assumptions
- 1 gallon of standard architectural paint covers approximately 350 to 400 square feet.
- 1 standard interior door accounts for 21 sq ft; 1 standard window accounts for 15 sq ft.
Frequently asked questions
How many square feet does 1 gallon of paint cover?
One standard gallon of quality latex interior paint covers approximately 350 to 400 square feet of smooth drywall with a single coat.
Do I always need 2 coats of paint?
Yes, 2 coats are strongly recommended for even color depth, durable washability, and complete hide, especially when making drastic color shifts or painting over fresh primer.
How much paint is needed for a ceiling?
Calculate ceiling area as Length × Width. A 12 × 14 ft ceiling (168 sq ft) requires approximately 0.5 gallons (or 2 quarts) per coat.
When is primer required before painting?
Primer is required on unpainted drywall, patched spackle areas, raw wood, glossy oil-based paint, or when covering severe water/smoke stains.
What is the difference between matte, eggshell, and satin finishes?
Matte/flat hides wall imperfections and is ideal for ceilings and low-traffic bedrooms. Eggshell and satin have a soft sheen that resists scuffs and moisture, making them ideal for living rooms, hallways, and kitchens.
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